1. Field of the Invention
The present invention generally relates to a solid-state infrared imager, and more particularly to an uncooled or thermal solid-state infrared imager having an improved signal-readout structure.
2. Description of the Related Art
An infrared imager is capable of capturing the image of an object without any distinction between night and day, and is advantageous in that the image is captured using infrared radiation whose penetration in smoke or fog is higher than that of visible radiation and in that temperature information of the object is also obtained. Therefore, the imager is widely applicable to defense systems, surveillance cameras, fire detection cameras, and the like.
Conventionally, the solid-state infrared imager of the quantum type has been regarded as the mainstream imager, but has a serious drawback in that a cooling system is required for low-temperature operation. In recent years, development of an uncooled solid-state infrared imager, which does not require a cooling system, has become vigorous. The uncooled solid-state infrared imager obtains infrared image information, read as an electric signal corresponding to incident infrared radiation having a wavelength of about 10 μm, from each thermal or infrared sensing pixel using a structure for absorbing the infrared radiation as heat which causes a slight change in the temperature thereof, and using a thermoelectric converter to convert the temperature of the absorbing structure to an electric signal.
As an example of an infrared sensing pixel of the uncooled solid-state infrared imager, an infrared sensor containing a silicon pn-junction thermoelectric converter element has been reported (Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999). This thermoelectric converter element is formed in an SOI substrate to convert the temperature change of an infrared absorber to a voltage change using a constant forward-bias current. The silicon pn-junction thermoelectric converter element using the SOI substrate has the advantageous feature that the element can be fully manufactured by a silicon LSI manufacturing process, and is therefore superior in mass producibility. Moreover, the pn-junction thermoelectric converter element has pixel selectivity inherent in rectification characteristics. Therefore, the internal structure of each pixel can be simplified with the use of this converter element.
Additionally, the temperature change of an infrared sensing pixel in the uncooled solid-state infrared imager depends on the absorptance of an infrared absorber or an optical system, but is generally about 5×10−3 times the temperature change of the object. When the object temperature changes by 1 K, the pixel temperature changes by 5 mK. In a case where eight silicon pn-junction diodes are connected in series to form the pn-junction thermoelectric converter element for each pixel, thermoelectric conversion efficiency is about 10 mV/K. Therefore, when the object temperature changes by 1 K, a signal voltage of 50 μV is generated in each pixel. Actually, a resolution for distinguishing the object temperature difference of about 0.1 K is required in many cases. Therefore, it is necessary to read a signal voltage of about 5 μV generated for the temperature difference.
As a method of reading out this very slight signal voltage, there is a known circuit in which the signal voltage generated in each pixel is used as a gate voltage of a MOS transistor for current-amplification and the amplified signal current is integrated over time by an integration capacitor. This circuit is called a gate modulation integrator circuit (GMI circuit). Such GMI circuits are disposed as column amplifiers in columns of a pixel matrix array to amplify the currents of the pixels in one row in parallel. This structure limits the signal bandwidth. Thus, random noise can be reduced.
The voltage gain G in the gate modulation integrator circuit is principally determined by the mutual conductance gm (=δId/δVg) of the amplification transistor, the integration time ti, and the integration capacitance Ci, and is represented as follows:G=(ti×gm)/Ci  (1)When the integration time ti and integration capacitance Ci are given, the above-described gain is determined by the mutual conductance of the amplification transistor gm. When the n-type MOS transistor operates in a saturated region, gm is approximated by the following equation (2):gm=(W/L)·(εox/Tox)·μn·(Vgs−Vth)  (2)where W is the channel width, L is the channel length, εox is the permittivity of the gate oxide film, Tox is the gate oxide film thickness, μn is the electron mobility, Vgs is the voltage between gate and source, and Vth is the threshold voltage of the transistor.
As described above, a resolution for distinguishing the object temperature difference of about 0.1 K is required. Therefore, it is necessary to read a signal voltage of about 5 μV when the signal voltage is output from a pixel. This signal voltage level is very low compared with the voltage of a CMOS sensor by which an image is captured with general visible radiation. For example, according to a document (“High-Sensitivity CMOS Image Sensor”, the Journal of the Institute of Image Information and Television Engineers Vol. 54, No. 2, p. 216, 2000, the entire contents of which are incorporated by reference), the noise voltage is about 0.4 mV=400 μV. Compared with this, the noise level of the above-described infrared sensor is a low voltage corresponding to about 1/80 of the level of the CMOS sensor, and the signal voltage to be handled is similarly a low voltage of about 1/80.
Therefore, considering that an infrared sensor output is processed by circuitry similar to the CMOS sensor, the column amplifier must be formed of a gate modulation integrator circuit having about 80 times the gain.
The above-described uncooled solid-state infrared imager uses circuitry called a constant current biased voltage readout system to read out a signal from an infrared sensor containing a silicon pn-junction thermoelectric converter element. This solid-state infrared imager has some problems.
As a first problem, a plurality of silicon pn-junction diodes need to be connected in series inside each pixel in order to enhance the thermoelectric conversion sensitivity. Therefore, the pixel structure becomes complicated, and it is difficult to miniaturize the pixels.
A second problem results from the first problem. Since the infrared sensor uses the plurality of silicon pn-junction diodes connected in series within the pixel, a voltage much higher than a usual CMOS device power source voltage of 1 to 3 V is required for an optimum operation of the infrared sensor.
For example, when the number of pn-junction diodes is eight, a high voltage close to 10 V is necessary. Therefore, a design and manufacturing process of peripheral circuits such as a row selection circuit requires additional structure and manufacturing process for increasing a withstand voltage, which are not required in a standard CMOS device.
A third problem exists in the thermoelectric conversion sensitivity. Considering from the operation principle of the uncooled solid-state infrared imager using the silicon pn-junction thermoelectric converter element, the thermoelectric conversion sensitivity of the constant current biased voltage readout system is lower than that of a constant voltage biased current readout system.
Furthermore, another important problem is a self-heating problem. The uncooled solid-state infrared imager generally requires a current flowing in the thermoelectric converter element in order to read out temperature information from the thermoelectric converter element as an electric signal. Joule-heating is generated in the thermoelectric converter element by a bias current or voltage for reading out the temperature information, and the thermoelectric converter element is heated by this Joule-heating. A so-called self-heating problem occurs in this manner.